1. Technical Field
The present invention relates to the handling and processing of semiconductor wafers. More particularly, the present invention relates to loading and unloading a cassette of wafers in a wafer processing chamber.
2. Description of the Prior Art
Semiconductor wafer handling and processing have always required extreme care to avoid contamination due to ambient and process borne particles. Such contamination lowers process yields, thus making the manufacture of integrated circuits more expensive and time consuming. Thus, as device features of less than 0.5 .mu.m become prevalent, a clean room environment of less than class 1, i.e. an environment having less than one contaminant particle of dimension less than or equal to 0.5 .mu.m per 0.3 m.sup.3 (ft.sup.3) of clean room volume, is generally required.
At the present level of integration, semiconductor processes are critically sensitive to any contamination. Accordingly, one goal of semiconductor manufacturers is to avoid as much as possible exposing semiconductor wafers to sources of contamination. Class 1 conditions require that processed or partially processed wafers must be bagged when transported to subsequent processing facilities or to outside analysis or other systems. Industry practice is to place semiconductor wafers in spaced stacks, referred to as cassettes, and to place these cassettes into various types of microenvironments, such as sealed containers, for example in the standard mechanical interface (SMIF) box of the type manufactured by Asyst Technologies, Inc. of Milpitas, Calif. Such SMIF-type boxes provide a sealed environment in which the cassette is readily transported under conditions that tend to minimize the potential for wafer contamination. The interior of the SMIF-type box is typically sealed by placing a removable base or lid on the box to form an integrated, airtight assembly. Withdrawal of the cassette from the SMIF-type box for wafer loading/unloading is accomplished by several different methods, for example a pressure differential may be provided at the base of the SMIF-type box, such that the base and cassette of wafers is pulled from the SMIF-type box, or the base of the SMIF-type box may be mechanically engaged with a platform and/or support, etc. In this way, wafer exposure to the ambient environment is reduced during wafer handling, thus limiting the possibility that particles can fall onto and contaminate the wafers. See, for example Bonora et al (U.S. Pat. No. 4,995,430) and Mortensen et al (U.S. Pat. Nos. 4,709,834 and 4,582,219).
Another opportunity for wafer contamination occurs when the wafers are loaded to and unloaded from processing chambers during the manufacture of integrated circuits. During such operation, residual particles are stirred and circulated as the processing chamber is pumped and/or vented to establish operating pressures or to restore loading pressures. The arrangement of the interface between the processing chamber and the wafer carrier, i.e. the load lock, is critical to maintaining a clean environment within the processing chamber because the load lock is one point at which particles frequently have the opportunity to enter the process chamber.
Examples of a prior art wafer transfer mechanisms include Iwasawa et al (U.S. Pat. No. 4,826,360), in which a transfer vehicle carrying a wafer pod is shuttled through a transfer tube by application of a negative pressure within the tube; Bonora et al (U.S. Pat. No. 4,995,430) which is discussed above; Davis et al (U.S. Pat. No. 5,044,871) in which a nonstandard bell-shaped carrier is maintained under vacuum for wafer transport; Crabbet al (U.S. Pat. No. 5,092,728) in which wafers are loaded into a receiving chamber to permit purging; Cruz et al (U.S. Pat. No. 5,112,277) in which wafers are loaded into a purging chamber; Wagner et al (U.S. Pat. No. 4,990,047) in which multiple wafers are handled within a process chamber; Matsushita et al (U.S. Pat. No. 5,058,526) in which a CVD apparatus includes an internal wafer and cassette transfer mechanism; Tullis et al (U.S. Pat. Nos. 4,532,970; 4,534,389; and 4,705,444) in which an atmospheric wafer carrier is docked to a load lock in a process chamber.
In Davis the wafers are carried upside down, which requires that the wafers be processed using unconventional techniques. Both Bonora and Davis place a sealed wafer container in a first chamber which encloses the container. Once the first chamber is sealed and evacuated, the wafer container is opened and the wafer cassette contained therein is transferred to a second chamber. Each wafer is then transferred from the second chamber to a process chamber where the wafer is processed. The wafer is then returned to the wafer cassette and the next wafer is selected and processed, and so on until every wafer in the cassette has been processed.
Crabb and Cruz both require a purge step before wafer transfer can occur. While purging may evacuate particles from the process environment before wafer loading, purging also stirs up residual particles such that those particles not evacuated during purging are likely to settle on the surface of the wafers within the processing chamber.
Thus, the state of the art is such that wafers contained within a wafer carrier must be loaded to or unloaded from a process chamber by an intermediate chamber into which the wafer cassette is first placed and which is then purged prior to wafer loading. Each step associated with interfacing the carrier to the process chamber takes time, subjects the wafers to excessive handling, and provides opportunities for wafer contamination. A system for directly loading wafers to a process chamber would improve process throughput, increase device yield per wafer, and thereby reduce manufacturing cost.